I have a Galaxy S I9000 (2.3.6, Gingerbread.XXJW4) and use my phone minimally (mostly just on standby). I would like a least a couple of days, and hopefully a week or two between recharges if possible (like my old crappy phone with a smaller battery).
There's already been reports that lowering the CPU frequency using something like SetCPU (needs root) can increase battery life, along with apps like 'Disable Service' to freeze stubborn apps. And this thread gives some sound advice for reducing battery usage further.
But I'm wondering if a custom ROM can bring battery life conservation up a level. I've been researching for the lightest ROM, and one called 'Simplicity' seems the most promising (Slim Bean, CyanAOSP, Vanilla RootBox, DarkyROM and C-RoM Mix are other contenders). However there are also reports that the stock ROM is best for battery life, in which case none of these custom ROMS would be worth bothering with for me. On the other hand, many people in that thread are reporting great battery life, with some lasting over 2 or even 3 days (I currently get just over half a day).
I am mostly thinking about processes/services such as "Android System" and "Android OS", as they rise to the top of CPU usage for me. Would these have the potential to be lighter on the battery with a custom ROM compared to the stock ROM on my I9000? How about "Wi-Fi", "Cell standby", "gpsd", or even "Phone idle"?
More information is found here, though it's not quite the same question, and I'm not fully convinced with the highest rated answer there yet (not mentioning CPU reclocking or aforementioned processes such as a lighter "Android System" process potentially).